Welcoming gallium- and indium-fumarate MOFs to the family: synthesis, comprehensive characterization, observation of porous hydrophobicity, and CO₂ dynamics

The properties and applications of metal-organic frameworks (MOFs) are strongly dependent on the nature of the metals and linkers employed, along with the specific conditions employed during synthesis. Al-fumarate, trademarked as Basolite A520, is a porous MOF that incorporates aluminum centers along with fumarate linkers, and is a promising material for applications involving adsorption of gases such as CO₂. In this work, the solvothermal synthesis and detailed characterization of the gallium and indium fumarate MOFs (Ga-fumarate, In-fumarate) are described. Using a combination of powder X-ray diffraction, Rietveld refinements, solid-state NMR spectroscopy, infrared spectroscopy, and thermogravimetric analysis, the topologies of Ga-fumarate and In-fumarate are revealed to be analogous to Al-fumarate. Ultra-wideline ⁶⁹Ga, ⁷¹Ga and ¹¹⁵In NMR experiments at 21.1 T strongly support our refined structure. Adsorption isotherms show that the Al-, Ga-, and In-fumarate MOFs all exhibit an affinity for CO₂, with Al-fumarate the superior adsorbent at 1 bar and 273 K. Static direct excitation and cross-polarized ¹³C NMR experiments permit investigation of CO₂ adsorption locations, binding strengths, motional rates, and motional angles that are critical to increasing adsorption capacity and selectivity in these materials. Conducting the synthesis of the indium-based framework in methanol demonstrates a simple route to introduce porous hydrophobicity into a MIL-53-type framework, by incorporation of metal-bridging -OCH₃ groups in the MOF pores.